This invention relates to techniques for polycrystalline silicon (polysilicon) deposition for the fabrication of a semiconductor integrated device, and, more particularly, to the formation of a layer of polysilicon on a dielectric layer.
The scaling down toward ever smaller integrated structures and the consequent reduction of operating voltages, as well as of the thickness of dielectric layers for capacitive coupling, aggravates the criticality of residual imperfections in the dielectric. These imperfections occur during the process of formation of the dielectric layer and/or are induced in the dielectric film during the successive steps of fabrication of the integrated device that includes several different thermal treatments.
In highly critical dielectric layers, like for instance in the case of the tunnel oxide in a FLASH or EEPROM memory structure for the capacitive coupling between a floating gate patterned from a deposited polycrystalline silicon layer, eventually doped in-situ, and a drain region of the underlying monocrystalline silicon semiconductor, it is generally accepted that the dielectric properties may be affected by the characteristics of the layers interfacing or being coupled therewith. An interface layer may typically be polycrystalline silicon which is the conductor material interfaced to the dielectric film. Another critical dielectric layer is the so-called xe2x80x9cinterpolyxe2x80x9d dielectric layer for the capacitive coupling between a floating gate patterned from a first level polycrystalline silicon and a control gate patterned from a second level polycrystalline silicon. This influence manifests itself during thermal treatments taking place after the formation of these stacked layers and depends from the way these layers were formed (temperatures, pressure, gas flow rates, residence times, growth rate).
With identical fabrication conditions, it has been observed that the reliability of a dielectric layer for instance of a tunnel oxide, in terms of breakdown, is markedly influenced by certain characteristics of the electrically conducting layer deposited over the dielectric. Surprisingly, it has been found that a layer of tunnel oxide (SiO2) performs better the larger the grain size or the size of single crystal domains or crystallites of the conducting material deposited over the oxide layer. Such a conducting material is typically a doped polycrystalline silicon (polysilicon) layer.
It has been established further that a highly efficient structure is defined by interfacing, to a dielectric layer for capacitive coupling, an electrically conductive layer of doped polycrystalline silicon or equivalent electrically conductive material, having crystallization grains whose dimensions are substantially of the same size of the area of capacitive coupling to be defined. This means that, on such a coupling area, the conductive layer is of an ideally unigranular polycrystalline material or that there exists an extremely small number of distinct crystalline domains or crystallites.
It has been found that such a crystalline structure, for example of polycrystalline silicon, may be obtained by depositing an essentially amorphous silicon film at a sufficiently low temperature and crystallizing the deposited film by an annealing step conducted at a sufficiently low temperature lower than the deposition temperature, typically below 650xc2x0 C. The technological research aimed to fulfill such a requirement through economically viable techniques compatible with productivity requirements, has led to establish that is it possible to form layers of polycrystalline silicon, doped or undoped, with grains of dimensions comparable to those of the relevant structural features that are defined in modern fabrication processes of integrated circuits. This may be done by using a chemical vapor phase deposition reactor capable of being operated at relatively high pressures and with controlled rapid temperature variations.